1. Field of the Invention
The invention relates to spectrometry in a downhole well bore environment and specifically, it pertains to a robust apparatus and method for determining an optimal pumping rate based on a in situ downhole dew point pressure or bubble point pressure either known or determined by measuring light spectra for electromagnetic absorbance for a formation fluid sample while decreasing the pressure on the sample under test.
2. Summary of the Related Art
Earth formation fluids present in a hydrocarbon producing well typically comprise a mixture of oil, gas, and water. The pressure, temperature and volume of formation fluids control the phase relation of these constituents. In a subsurface formation, high well fluid pressures often entrain gases within the oil above the bubble point pressure. When the pressure is reduced, the entrained or dissolved gaseous compounds separate from the liquid phase sample. The accurate measure of pressure, temperature, and formation fluid composition from a particular well affects the commercial interest in producing fluids available from the well. The data also provides information regarding procedures for maximizing the completion and production of the respective hydrocarbon reservoir.
Certain techniques analyze the well fluids downhole in the well bore. U.S. Pat. No. 6,467,544 to Brown, et al. describes a sample chamber having a slidably disposed piston to define a sample cavity on one side of the piston and a buffer cavity on the other side of the piston. U.S. Pat. No. 5,361,839 to Griffith et al. (1993) disclosed a transducer for generating an output representative of fluid sample characteristics downhole in a wellbore. U.S. Pat. No. 5,329,811 to Schultz et al. (1994) disclosed an apparatus and method for assessing pressure and volume data for a downhole well fluid sample.
Other techniques capture a well fluid sample for retrieval to the surface. U.S. Pat. No. 4,583,595 to Czenichow et al. (1986) disclosed a piston actuated mechanism for capturing a well fluid sample. U.S. Pat. No. 4,721,157 to Berzin (1988) disclosed a shifting valve sleeve for capturing a well fluid sample in a chamber. U.S. Pat. No. 4,766,955 to Petermann (1988) disclosed a piston engaged with a control valve for capturing a well fluid sample, and U.S. Pat. No. 4,903,765 to Zunkel (1990) disclosed a time delayed well fluid sampler. U.S. Pat. No. 5,009,100 to Gruber et al. (1991) disclosed a wireline sampler for collecting a well fluid sample from a selected wellbore depth, U.S. Pat. No. 5,240,072 to Schultz et al. (1993) disclosed a multiple sample annulus pressure responsive sampler for permitting well fluid sample collection at different time and depth intervals, and U.S. Pat. No. 5,322,120 to Be et al. (1994) disclosed an electrically actuated hydraulic system for collecting well fluid samples deep in a wellbore.
Temperatures downhole in a deep wellbore often exceed 300 degrees F. When a hot formation fluid sample at 300 degrees F. is retrieved to the surface at a temperature of 70 degrees F., the resulting decrease in temperature causes the formation fluid sample to contract. If the volume of the sample is unchanged, such contraction substantially reduces the sample pressure. A pressure drop can result in changes in the situ formation fluid parameters, and can permit phase separation between liquids and gases entrained within the formation fluid sample. Phase separation significantly changes the formation fluid characteristics, and reduces the ability to evaluate the actual properties of the formation fluid.
To overcome this limitation, various techniques have been developed to maintain pressure of the formation fluid sample. U.S. Pat. No. 5,337,822 to Massie et al. (1994) pressurized a formation fluid sample with a hydraulically driven piston powered by a high-pressure gas. Similarly, U.S. Pat. No. 5,662,166 to Shammai (1997) used a pressurized gas to charge the formation fluid sample. U.S. Pat. Nos. 5,303,775 (1994) and U.S. Pat. No. 5,377,755 (1995) to Michaels et al. disclosed a bi-directional, positive displacement pump for increasing the formation fluid sample pressure above the bubble point so that subsequent cooling did not reduce the fluid pressure below the bubble point.
Existing techniques for maintaining the sample formation pressure are limited by many factors. Pretension or compression springs are not suitable because the required compression forces require extremely large springs. Shear mechanisms are inflexible and do not easily permit multiple sample gathering at different locations within the well bore. Gas charges can lead to explosive decompression of seals and sample contamination. Gas pressurization systems require complicated systems including tanks, valves and regulators which are expensive, occupy space in the narrow confines of a well bore, and require maintenance and repair. Electrical or hydraulic pumps require surface control and have similar limitations.
If during pumping a sample into a sample tank, the pressure drops below the bubble point pressure or dew point pressure, nucleation of gas bubbles, precipitation of solids, and hydrocarbon loss respectively changes the single-phase liquid crude sample into a two-phase or three phase state consisting of liquid and gas or liquid and solids. Single phase samples which represent the native state of the formation fluid are sought for analysis of the formation in downhole conditions. Two-phase samples are undesirable, because once the crude oil sample has separated into two phases, it can be difficult or impossible and take a long time (weeks), if ever, to return the sample to its initial single-phase liquid state even after reheating and/or shaking the sample to induce returning it to a single-phase state.
Due to the uncertainty of the restoration process, any pressure-volume-temperature (PVT) lab analyses that are performed on the restored single-phase crude oil are of suspect quality and consistency. Thus there is a need for a process for determining the dew point for a formation sample so that an optimal pumping rate can be selected while sampling to ensure that the pressure does not drop below the dew point or bubble point pressure during sampling and risk sample spoilage.